This invention relates generally to special purpose motor vehicles and has particular pertinence to a vehicle for cleaning the road or street by means of a recirculating airflow while traveling. Still more specifically, the invention deals with improved means on such a road-cleaning vehicle for effecting separation of dust from the recirculating air.
U.S. Pat. No. 3,545,181 represents a typical prior art air-dust separation system for road-cleaning vehicles. This prior art system teaches a centrifugal air-dust separation chamber having a cylindrical shape including an open end directed toward a blower, which produces a powerful airflow by drawing air from the separation chamber. This chamber has an inlet opening which extends parallel to its axis and which is so defined and arranged that the dust-laden air in the rubbish hopper of the vehicle is drawn substantially tangentially into the chamber by virtue of a partial vacuum created therein by the blower. Therefore, as the dustladen air spirals in the chamber and flows toward the blower, the dust particles are centrifugally forced out of a dust outlet of the chamber which is in direct communication with the hopper.
The above-stated conventional air-dust separation system has several drawbacks. One of these arises from the fact that the dust outlet of the air-dust separation chamber is open directly to the rubbish hopper. Since the blower creates a partial vacuum in the air-dust separation chamber, the dust-laden air has been drawn therein not only through the air inlet but also through the dust outlet. Accordingly, the only dust particles actually expelled through the dust outlet are those which have sufficient mass to overcome the force of the undesired inflowing air. The dust particles having a smaller mass that have not been freed from the recirculating air are drawn toward the blower, with some of such particles striking the impeller of the blower. The dust particles striking the blower can, indeed, be likened to the abrasive powder used in shot blasting, so rapid has been the abrasion of the impeller. Furthermore, as the impeller itself centrifugally hurls the dust particles away, the housing of the blower has also been subjected to rapid wear. This problem becomes all the more serious when the vehicle is cleaning sandy or gravelly roads. In the worse case known to the applicant, the useful life of the blower and its housing was only about one month.
Another inconvenience caused by the prior art road-cleaning vehicles of the type under consideration has been the pollution of atmospheric air with the dust discharged therefrom. The usual practice with road-cleaning vehicles is to admit fresh atmospheric air into the pickup head from its front side at a rate depending upon the prevailing kind of rubbish to be collected, with the concurrent partial exhaustion of the recirculating air at a matching rate. For such partial exhaustion of the recirculating air, an exhaust duct is mounted to the blower housing for communication with its interior via a regulator door. The rate of admission of atmospheric air can be controlled by regulating the rate at which the recirculating air is drawn off past the regulator door.
The air pollution problem particularly manifests itself when the rubbish to be collected is predominantly lightweight matter such as dead leaves or waste paper. For efficiently drawing up such lightweight rubbish, the regulator door is fully opened, with the consequent admission of atmospheric air, and exhaustion of the recirculating air, at the maximum rate. Such exhaust air has heretofore contained large amounts of fine dust particles that have not been separated from the recirculating air by the prior art means.
It has been suggested to avoid air pollution to employ a high pressure blower, in addition to that for air recirculation within the vehicle, exclusively for forcing the dust-laden air through a nonwoven fabric filter prior to exhaustion. The provision of such additional filtration means, including, of course, means for reconditioning the filter, is objectionable not only from the standpoint of the added costs for their manufacture and installation but also from that of the inevitable reduction of the hopper capacity.